Process and unit for supplying a gas under pressure to an installation that consumes a constituent of air

ABSTRACT

Air is distilled in a distillation apparatus (13) associated with a heat exchange line (11) in which is circulated a heat transfer fluid under high pressure. A storage (20, 23) is supplied, at least intermittently, with gas in liquid phase; and gas in liquid phase withdrawn from the storage is raised to a vaporization pressure, and vaporized under this pressure in the heat exchange line (11). The supply of liquefied gas to the storage can be from the distillation apparatus (13) and/or from an external source such as a tank truck (29, 34). It is useful particularly for the supply of oxygen, nitrogen and argon under pressure to installations for the production of stainless steel such as electric arc furnaces.

This application is a division of application Ser. No. 08/242,452, filed May 13, 1994.

The present invention relates to a process for supplying to a consuming installation at least one constituent of air from a gas under pressure liquefiable at a cryogenic temperature under this pressure.

The invention is applicable in particular to the supply of certain metallurgical installations with oxygen and/or nitrogen and/or argon.

The pressures in question hereinafter are absolute pressures.

Certain metaiurgical installations consume simultaneously gaseous oxygen under pressure, gaseous nitrogen under: pressure and gaseous argon under pressure, for example electric arc furnaces adapted to produce stainless steel (AOD process). The pressure of utilization of these gases is typically of the order of 25 bars, with very variable consumption, which requires the use of "buffers" or auxiliary storage supplied under a pressure of the order of 30 to 40 bars.

Oxygen and nitrogen are generally produced by an air distillation unit at the site, and the quantities of argon necessary are much greater than the production capacity of argon of this distillation unit, which cannot exceed 5 percent of the oxygen flow. It is therefore necessary to provide the installation with argon from an external source. In the prior art, this argon is vaporized in a supplemental apparatus such as a hot water bath.

Moreover, the evolution of the technology as well as the production requirements have as a consequence a progressive increase in consumption of oxygen in arc furnaces, which often ultimately greatly exceeds the separation capacity of the distillation apparatus at the site.

The invention has for its object to provide a process permitting satisfying in a particularly economical fashion the gas requirements, even very changeable, of the installation.

To this end, the invention has for its object a process of the above type, characterized in that:

air is distilled in a distillation apparatus associated with a heat exchange line in which is circulated at least one heat transfer fluid under a high pressure substantially greater than the operating pressure of the distillation apparatus;

a storage is supplied in an intermittent fashion with said gas in liquid phase; and

at least a portion of the gas under pressure is produced by bringing the gas in liquid phase withdrawn from the storage to a vaporization pressure higher than its storage pressure, and vaporizing it under this vaporization pressure in the heat exchange line associated with the distillation apparatus.

The process can comprise one or several of the following characteristics:

at least a portion of the storage is supplied from an external source;

the heat of vaporization/reheating of said gas is at least partially recuperated by producing an additional flow of liquid oxygen and/or liquid nitrogen;

there is produced simultaneously at least one constituent of air under pressure by bringing this constituent in liquid phase withdrawn from the distillation apparatus, to a vaporization pressure higher than its withdrawal pressure, and by vaporizing this liquid under this vaporization pressure in the heat exchange line;

said gas is an air gas whose flow rate is greater than the flow rate of this gas produced at the same moment by the distillation apparatus;

said vaporization is effected continuously;

during certain periods, there is sent from the distillation apparatus to the storage, gas in liquid phase, and, during other periods, said vaporization is performed.

The invention also has for its object a unit adapted to practice such a process. This unit, of the type comprising an air distillation apparatus associated with a heat exchange line which comprises passages for at least one heat transfer fluid under high pressure substantially greater than the operating pressure of the distillation apparatus, is characterized in that it comprises a storage of said gas in liquid phase adapted to be supplied at least intermittently, and means for compressing this gas in liquid phase connected upstream of this storage and downstream of the vaporization passages for this gas under pressure in the heat exchange line.

The unit can comprise one or several of the following characteristics:

compression means can also be connected downstream of an auxiliary vaporization device;

said gas being an air gas, the compression means are connected only to the storage, this latter being if desired connected to the distillation apparatus;

the unit comprises a pump for said gas in liquid phase, whose inlet is connected to the distillation apparatus;

the output of the pump is connected to vaporization passages of the heat exchange line separate from those associated with the compression means.

Examples of embodiment of the invention will now be described with respect to the accompanying drawings, in which:

FIG. 1 represents schematically a unit according to the invention; and

FIGS. 2 and 3 show schematically and partially two modifications.

There is shown in FIG. 1 a unit adapted to supply to a consumption installation 1, which is typically a metallurgical installation practicing a process of the AOD type, on the one hand gaseous oxygen under pressure, carried by a conduit 2, on the other hand gaseous nitrogen under pressure, carried by a conduit 3, and finally argon under pressure, carried by a conduit 4. This unit also produces low pressure gaseous nitrogen, which is to say adjacent atmospheric pressure, as well as liquid oxygen and liquid nitrogen, these liquids being also under roughly atmospheric pressure.

The unit consists essentially in an atmospheric air compressor 5; a refrigeration group 6; a device 7 for drying and carbon dioxide removal from the air by absorption; a blower 8 and an expansion turbine 9 whose rotors are keyed on the same shaft, the blower being provided with an air or water refrigerant 10; a heat exchange line 11 of the countercurrent type; a subcooler 12; a distillation apparatus 13 constituted by a double column of the "minaret" type, comprising a medium pressure column 14 which operates at about 5 to 6 bars and which is surmounted by a low pressure column 15 operating at about 1.2 bar, the head of the column 14 being coupled to the base of column 15 by an evaporator-condenser 16; a phase separator 17; a storage 18 for liquid nitrogen; a liquid oxygen pump 19; a storage 20 for liquid oxygen; a liquid nitrogen pump 21; the liquid argon pump 22; and a storage 23 for liquid argon. The three storages 18, 20 and 23 are at substantially atmospheric pressure.

In operation, atmospheric air to be distilled, compressed in 5 and then cooled in 6, is purified of water and carbon dioxide in 7 and then further compressed in its entirety in 8 and brought to the vicinity of ambient temperature in 10. The further compressed air, which is at a high pressure substantially greater than the medium pressure of column 14, is cooled in its totality to an intermediate temperature T in the heat exchange line 11. At this temperature, a portion of the air is cooled and is liquified and if desired subcooled by passage to the cold end of the heat exchange line, then, after expansion in expansion valves 24 and 25, divided between the two columns 14 and 15.

At temperature T, the rest of the air is expanded to medium pressure in 9 and then introduced into the base of column 14.

There is also indicated in the drawing the conventional conduits and double minaret conduits, which is to say producing at the head of the low pressure column pure nitrogen under this low pressure. The conduits 26 to 28 for raising respectively "rich liquid" (air enriched in oxygen), "lower poor liquid" (impure nitrogen) and "upper poor liquid" (practically pure nitrogen), emerge respectively from the base, an intermediate point in the head of column 14, passing through the subcooler 12 and being each provided with an expansion valve.

The oxygen destined for the installation 1 is withdrawn in liquid form from the base of the column 15, brought to the utilization pressure by the pump 19, and vaporized and reheated to about ambient temperature in the heat exchange line.

Similarly, the nitrogen destined for installation 1 is withdrawn in liquid phase from the head of the column 14, brought to its utilization pressure by the pump 21, and vaporized and reheated to about ambient temperature in the heat exchange line.

Argon destined for the installation 1 is stored in liquid phase in storage 23, supplied by tank trucks 29. The liquid argon is withdrawn from this storage, brought to its utilization pressure by the pump 22, and vaporized and reheated to about ambient temperature in the heat exchange line.

In a conventional manner, the impure nitrogen constituting the residual gas of the installation and the pure nitrogen product at the head of column 15 are reheated in 12 and then in 11 and leave the unit, respectively, via conduits 30 and 31.

The unit produces moreover liquid nitrogen, withdrawn by the conduit 28 and expanded in an expansion valve 32 before being introduced into the phase separator 17. The liquid from this phase separator is sent to storage 18, while the vapor phase is combined with the gaseous nitrogen flow from the top of column 15. The unit also produces liquid oxygen, withdrawn from the base of column 15 and sent to storage 20 via a conduit 33. As will be understood, the additional quantity of cold introduced into the heat exchange line by the vaporization and reheating of the liquid argon is recovered to produce liquid oxygen and/or liquid nitrogen.

As a modification, the air distillation apparatus can comprise a column for argon production coupled in conventional manner to the column 15, so as to produce a portion of the argon supply to the installation 1.

FIGS. 2 and 3 relate to the supply of the installation with gaseous oxygen under pressure at a flow rate greater than the separation capacity of the distillation apparatus 13. In these figures, all that has been shown is the double column 13 and the oxygen circuit; the rest of the installation is the same as in FIG. 1, except for the argon circuit, which is omitted.

In the case of FIG. 2, the storage 20 is supplied on the one hand from the base of column 15, and on the other hand by tank trucks 34, while the inlet of the pump 19 is connected to the base of the storage 20. Thus, the necessary flow rate of liquid oxygen can be supplied to the heat exchange line to be vaporized and reheated. Excess liquid oxygen vaporized, which is to say the quantity supplied by the tank trucks, supplies the cold which can be as before recovered in the form of additional liquid nitrogen sent to storage 18.

As indicated in FIG. 3, as a modification, the diagram of FIG. 2 can be completed by a second liquid oxygen pump 36 whose inlet is connected to the base of column 15, the outputs of the two pumps can be connected to the same vaporization passages of the heat exchange line, or even to separate vaporization passages.

As indicated in mixed lines, the output of the pump 19 and/or 36 can also, in case of need, for example for safety's sake, be connected to a conventional auxiliary vaporization apparatus 35 such as a bath. In this manner of operation, the cold contained in the vaporized liquid oxygen is of course lost.

In each of the cases of FIGS. 2 and 3, the distillation apparatus can progressively fill the storage 20, during periods of low demand for oxygen by the consuming installation and/or during periods of low electrical energy cost, while this storage progressively empties during periods of high demand and/or of high cost of electrical energy and must then if desired be supplied from tank trucks. If the oxygen demand is permanently higher than the capacity of the distillation apparatus, the supply by tank trucks must of course be effected regularly.

As a modification, the storage 20 in FIG. 3 may not be connected to the double column 13 and may be supplied only by tank trucks 34. In this case, the purity of the oxygen delivered by these latter can be different from that of the oxygen produced by the double column.

The invention is also applicable to an air distillation apparatus comprising a single column, for example an argon vaporization arrangement such as that of FIG. 1 but in which the apparatus 13 is a simple production column for nitrogen of the HPN (High Purity Nitrogen)type, associated with a Claude air cycle analogous to that described above. In this case, the vaporization of the argon permits producing an excellent liquid nitrogen. 

What is claimed is:
 1. Cryogenic distillation separation unit for producing at least one gaseous product comprising at least one distillation column, a vessel for storing a cryogenic liquid, pressurizing means for pressurizing said liquid, a main heat exchanger permitting heat exchange between a feed gas of the distillation column and said pressurized liquid, an auxiliary heat exchanger permitting vaporization of said pressurized liquid, and means for selectively sending said pressurized liquid to one of said main and auxiliary heat exchangers.
 2. A unit according to claim 1, comprising means for supplying liquid to said vessel from an external source or from said distillation column.
 3. A unit according to claim 1, wherein said pressurizing means is downstream of said vessel.
 4. A unit according to claim 1, further comprising further pressurizing means that receives a liquid directly from the distillation column, and means for selectively sending the last-mentioned liquid from the further pressurizing means selectively to one of the main and auxiliary heat exchangers.
 5. A unit according to claim 4, further comprising means for sending liquid from each of said pressurizing means to separate vaporization passages of said main heat exchanger.
 6. Cryogenic distillation unit for the separation of air, for producing at least one gaseous product, comprising at least one distillation column, a vessel for storing a cryogenic liquid, pressurizing means for pressurizing said liquid, a main heat exchanger permitting heat exchange between air and said liquid, an auxiliary heat exchanger permitting vaporization of said liquid, and means for selectively sending said liquid said auxiliary heat exchanger or directly to said main heat exchanger.
 7. Cryogenic distillation separation unit for producing at least one gaseous product, comprising at least one distillation column, a vessel for storing a cryogenic liquid, pressurizing means for pressurizing said liquid, a main heat exchanger located outside said column permitting heat exchange between a feed gas of the distillation column and said liquid, an auxiliary heat exchanger permitting vaporization of said liquid, and means for selectively sending said liquid to said auxiliary heat exchanger or directly to said main heat exchanger. 